Copy Transform Constraint Wrong Rotation
As mentioned by Denis, a child-of constraint would be easier.  But it's still possible to answer your explicit question, &quot;How can I set the empty rotation in such a way that the hand will have the correct resting rotation when I add the copy transform to the hand ik?&quot;

Get the hand posed how you'd like, without the copy transforms constraint.

Give the empty a copy transforms constraint (world-&gt;world space) targeting the hand bone.

Apply visual transform on the empty (ctrl A menu in object mode for me) then delete its copy transforms constraint.  If necessary, register keyframes for the empty.

Now give the hand bone the copy transforms constraint targeting the empty.